Add zero, one, two as private methods

* of course as module methods, they are not private ...
* so, not entirely sure if this is OK (it expands the module API
  of the BigMath module ...)
Remove some more C code from the guard code
Implement Arc Tang Hyperbolicus series for ln(x)

* still needs the scaling for very large and small values
* still needs more testing to validate the scenario of
  extreme precision (only manually tested to 15 digits
  precision now).